[dia] Fix crashing on DiaObject::get_object_menu() returning NULL

[Hans Breuer <hans src gnome org>]

commit aec5d45b0fe6382e11fc13ccf36d090da6d0a59c
Author: Hans Breuer <hans breuer org>
Date:   Tue Oct 2 20:53:48 2012 +0200

    Fix crashing on DiaObject::get_object_menu() returning NULL

 app/disp_callbacks.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/app/disp_callbacks.c b/app/disp_callbacks.c
index ffea0f1..5021f47 100644
--- a/app/disp_callbacks.c
+++ b/app/disp_callbacks.c
@@ -290,7 +290,7 @@ popup_object_menu(DDisplay *ddisp, GdkEventButton *bevent)
     num_items = 0;
   } else {
     dia_menu = (obj->ops->get_object_menu)(obj, &last_clicked_pos);
-    num_items = dia_menu->num_items;
+    num_items = dia_menu ? dia_menu->num_items : 0;
   }
 
   if (dia_menu->app_data == NULL) {